I highly recommend Snorkel Maui, 800-635-1273. There are three Marine Biologists who work out of there, Evelyn, Sue and Ann. Ask for Ann or Evelyn when you call (Sue was off the island when we were there so I did not talk with her). They offer private and group snorkel and dive trips. Evelyn took us to a snorkel spot that is closed to commercial use. It included a 45 minute walk through a lava field to get to it. There is not way we could have ever found this pool without her as a guide. It was by far the best spot we snorkeled on the entire trip. Plus she could share so much more about what we were seeing since she is a marine biologist. If they can not offer what you want, then they can send you in the right direction.
If you want their direct phone numbers, let me know and I'll PM them to you.